Soft solder is a fusible alloy, typically consisting of tin and lead, which is used for the purpose of joining together two or more metals at temperatures below their melting points. In addition to tin and lead, solders may occasionally contain varying amounts of other materials, such as antimony, bismuth, cadmium, or silver, which are typically added for the purpose of varying the physical properties of the alloy. However, in many solders, some of these elements, notably antimony, are only present as impurities. Solder is widely used in the electronics industry for attaching electrical components to printed circuit boards (PCB). In order to successfully attach the electrical components to the PCB, there must be a metallurgical affinity between the two metals that are going to be soldered together, the metals must be free from all non-metallic surface contamination, and there must be complete and adequate metallic contact between the solder and the metals that are to be soldered, and there must be a temperature adequate for sufficient alloying of the metal. The solder provides the attachment by virtue of an intermetallic solution, which takes place at the soldering temperature.
One method of assembling electronic assemblies utilizes solder preforms. Solder preforms, or solder forms, are pieces of solder formed into predetermined geometric shapes. The preforms are particularly suited to those operations which require a given or predetermined volume of solder to be applied to a relatively inaccessible portion of an assembly. In many cases, the parts are assembled with the solder preform in proper position and alignment. When heat is applied, the solder preform melts and the predetermined volume of solder is released at the desired point. Preforms lend themselves to virtually all types of heat application such as induction heat, conductive, and convective reflow. They are well adapted to mass production where the number of units that can be soldered at one time is limited only by the capacity of the reflow oven. The obvious advantage of solder preforms is the ability to precisely place a predetermined amount of solder directly on the PCB, without having to print solder paste. One of the disadvantages of this method is that the preforms are expensive and when reflowed on a bare PCB, it results in a domed surface on the solder pads. The domed surfaces degrade the accuracy of the subsequent component placement process. The parts and flux tend to slide off of the domed surface when the circuit board is subjected to accelerations while moving down the assembly line. Some have attempted to solve this problem by reducing the amount of solder used in the preform, thus decreasing the curvature of the domed surface. However, reducing the volume of solder decreases the total wetting forces of the solder on the pad and, therefore, reduces the tendency of the components to self-center during the assembly reflow operation. This self-centering tendency is extremely important to assure accurate alignment and high-yield soldering. In addition, one would like to maximize the amount of solder in order to create a full solder fillet between the component and the PCB.
Clearly, it would be a benefit to the electronics industry if a solder preform could be devised that includes the advantages of solder cladding, solder printing and conventional solder preforms.